.app {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}
.box {
  width: 400px;
  height: 70px;
  overflow: hidden;
  background-color: #edd;
}
.box li {
  height: 70px;
  width: 50px;
  float: left;
  position: relative;
}
.box li div {
  position: absolute;
  top: 0;
  left: 0;
  width: 50px;
  height: 140px;
}
.box li div img {
  float: left;
  display: block;
}
.box span {
  display: block;
  height: 70px;
  width: 50px;
  background: url('../img/b.jpg');
}
.box span.active {
  background: url('../img/c.jpg');
}
